An industry has grown and flourished in both advising and achieving the placing of telephone calls on a fraudulent basis through telephone switching systems, particularly in the toll interconnect network. Such an industry is known as "blue box" and "black box" fraud.
By way of illustration, a "blue box" call involves a user originating two calls, one legitimate, the other fraudulent, to a four-wire switching system office via a communication line utilizing in-band single frequency (SF) supervisory and multiple frequency (MF) address tones. While receiving an audible ringing indication for the legitimate call, the "blue box" user applies a SF tone to the call connections to simulate a disconnect of the legitimate call to the four-wire office. The "blue box" user immediately removes the SF tone which is recognized by the four-wire office as a new call origination and then is enabled effectively to dial the fraudulent call by MF signaling over the established connection through the four-wire office. At most, the "blue box" caller pays for the first legitimate call and places the second call at a reduced rate or free of charge. Typically, the "blue box" user will initially try to place the first call to a "800" number so that the "800" number is billed for both calls. When an "800" number is unavailable, the "blue box" user will then attempt to make a flat rate call such as to an information operator.
With "black box" calls, a user connects a "black box" to a line to receive calls and circumvent billing. The "black box" accomplishes this by generating an off-hook signal on a called line just long enough to trip ringing and establish a talking path, but short enough to prevent a valid call answer signal from being returned to the originating office to bill the user for the call. In addition, more sophisticated equipment such as a private branch exchange may be configured not to return an answer signal. Whatever the equipment or method, a fraudulent call involves either avoiding or altering the billing record and charges for the call.
Such fraudulent calls have stimulated the telephone industry, particularly interconnect carriers, to design systems for minimizing the effect of "black box", "blue box", and other fraudulent calls through the telephone network. Some such designs have included the use of auxiliary techniques and special equipment for sensing the SF-MF tones which are introduced into the network by fraudulent users and to disable the attempted misuse of telephone networks when a fraudulent call has been detected. However, such designs are typically effective only to detect and disable "blue box" originated calls outgoing from a toll interconnect network office, not "blue box" calls incoming to a toll interconnect network office or "black box" calls.
Furthermore, "blue box" users have attempted to make all of the special equipment busy by a multitude of calls and overloading the system to the extent that it must let through some of the fraudulent calls. The special equipment is costly and complex, delays the completion of calls, and often involves the need for substantial software or programming effort. It also requires the telephone company, particularly an interconnect carrier, to engineer an office for a larger call handling capacity than is actually needed.
A problem of the prior art, therefore, is that no single arrangement has been found for preventing both "blue box" and "black box" calls.